Art recognized processes for preparing palladium-on-carbon catalysts include the steps of forming a slurry from active carbon particles in an acid aqueous solution of a palladium compound; increasing the pH of said slurry to at least 12 by adding an alkali- or alkaline earth-metal hydroxide solution, as a result of which palladium hydroxide precipitates on the active carbon; reducing palladium hydroxide to metallic palladium, for instance, by adding to the slurry a reducing agent like formaldehyde, hydrazine or sodium formate (see U.S. Patent No. 3,736,266).
Inherent disadvantages of that known process are that a proper control of the addition of the reagents is to be heeded and that precipitation can only be carried out with the aid of alkali- or alkaline earth-metal hydroxides, since the required pH value is not achieved with carbonates and bicarbonates. Furthermore, if the preparation of the catalyst is undertaken according to that known process, the palladium salt, (after basic hydrolysis to Pd(OH).sub.2 or PdO.x H.sub.2 O, followed by reduction to Pd with, for instance, formaldehyde), is deposited in the form of coarse Pd metal crystallites whose active catalytic surface area is relatively small relative to the amount of metal precipitated. Coarse crystallites are understood to be crystallites having diameters exceeding 100 A. A relatively low activity is to be expected, therefore.
Another process, disclosed in U.S. Pat. No. 3,138,560, for preparing active palladium-on-carbon catalysts, is undertaken by impregnating basically reacting carbon with a palladium salt solution containing an oxidizing agent, for instance H.sub.2 O.sub.2, as a result of which palladium hydroxide or palladium oxidehydrate precipitates on the carbon; and then reducing the precipitate to metallic palladium. The oxidizing agent in this process serves to avoid reduction of the hydroxide or oxidehydrate during the hydrolysis of the palladium salt. Disadvantages of this process are that the carbon must be pretreated to provide a sufficient number of active basic sites on the carbon surface area, and that the palladium salt solution must invariably contain an oxidizing agent, though nevertheless, large Pd metal crystallites inevitably result.
U.S. Pat. No. 3,663,166 discloses preparation of platinum catalysts which are suitable for the reduction of NO to hydroxylamine by starting with a solution of a platinum salt in water; by adjusting the pH of the solution to a value of between 4.5 and 9 by means of an alkali-metal hydroxide or a alkali-metal carbonate; by subsequently adding the carbon carrier and heating the mixture so that hydrolysis occurs and Pt(OH).sub.4 precipitates on the carbon; whereupon, the mass is separated, washed and reduced with H.sub.2 in a 2-4 N sulphuric acid medium, or with formic acid in an aqueous suspension. The disadvantage of this process is that it is laborious and also gives rise to formation of coarse Pt metal crystallites.
The purpose of the invention is to provide a process for preparing highly active noble metal catalysts, in which the above objections, notably the laboriousness and/or the formation of coarse metal crystallites, are avoided.